Diode-lasers are commonly used as sources of illumination in various graphics applications such as display systems, optical printing systems and optical recording systems. In one type of prior-art imaging system, a linear (one-dimensional) array of light modulators is illuminated by an illuminator including a light source. Illumination from the array of modulators is projected onto a recording medium or the like in the form of a line of images of the modulators. By scanning the recording medium past the line of images and appropriately synchronizing the scanning with operation of the modulator array, the modulator array is used to draw a two-dimensional image on the recording medium.
A preferred light-source for the illuminator is a linear array of diode-lasers commonly referred to as a diode-laser bar. A diode-laser bar can efficiently provide relatively high illumination power, for example 60.0 Watts (W) or more, from a source having a maximum overall dimension no greater than about 10 centimeters (cm). A disadvantage, however, is that each of the diode-lasers in the bar is an individual emitter. This presents problems in selecting an appropriate optical configuration for the illuminator.
This problem is addressed in one prior-art illuminator by including a diode-laser array (or correspondingly an array of light-emitting diodes) having a number of emitters equal to the amount of modulators to be illuminated, and an optical system configured to image each emitter onto a corresponding modulator. This type of illuminator has a disadvantage that it is dependent on all emitters continuing to function. Failure of one emitter could cause at least a reduction in performance of the recording system, for example, a black line on an image in the direction of scanning.
An illuminator apparatus for overcoming this disadvantage is disclosed in U.S. Pat. No. 5,517,359 (Gelbart et al.). Here an optical system is used to project elongated overlapping images of each individual emitter onto a modulator array. The number of emitters need not correspond to the number of modulators, and failure of an individual emitter is described as merely reducing the overall illumination on the modulator rather than effecting the spatial distribution of illumination on the modulator.
Another illuminator apparatus for overcoming this disadvantage is described in a U.S. Pat. No. 6,494,371, assigned to the assignee of the present invention by inventors Rekow et al. The Rekow et al. arrangement comprises a diode-laser bar including a linear array of diode-lasers (emitters). An optical system is arranged with a longitudinal axis thereof parallel to the propagation direction of light from the diode-lasers. The optical system has mutually perpendicular axes designated the fast axis and the slow axis, and corresponding to fast and slow axes of the diode-lasers. The optical system is arranged to focus fast-axis diverging rays of the diode-lasers in a focal plane perpendicular to the longitudinal optical axis, and arranged to form the slow-axis diverging rays of the diode-lasers into a plurality of bundles of parallel rays, one thereof for each diode-laser. The bundles of parallel rays intersect in a focal plane of the optical system. The optical system thereby causes light from the diode-lasers to be formed into a line of light in the focal plane. The line of light has a width in the fast axis of the optical system and a length in the slow axis of the diode-lasers. In certain embodiments of the Rekow et al. apparatus, the distribution of intensity along the line of light is rendered somewhat more uniform by including one or more physical stops arranged to prevent selected portions the fast and slow-axis diverging rays from the diode-laser array from reaching the fast-axis focal plane of the optical system. The selected portions are chosen to optimize the intensity of light distribution along the length of the line of light. While somewhat effective in improving the intensity distribution along the line of light, the distribution is not believed to be optimum and comes at the expense of wasting light from the diode-laser array that is prevented by the physical stops from reaching the focal plane of the optical system.
U.S. patent application 20030128543, by Rekow, also assigned to the assignee of the present invention, discloses an illuminator apparatus that does not require the use of physical stops to achieve uniformity of illumination. An optical system focuses fast axis diverging rays from diode-lasers in a diode-laser array into a fast-axis focal plane of the optical system. The optical system forms slow-axis rays from the diode-lasers into bundles of parallel rays that intersect in the fast-axis focal plane. The focused fast-axis rays and the intersecting bundles of parallel rays form the line of light in the fast-axis focal plane. The optical system includes two arrays of cylindrical microlenses. The microlenses are spaced apart by a distance different from the spacing distance of diode-lasers in the diode-laser bar and are arranged with respect to the diode-laser bar such that each microlens in the array receives light from two or more of the diode-lasers in the diode-laser bar, and a plurality of microlenses receive light from two or more diode-lasers. This provides a line of light having a light uniformity comparable to that disclosed in the '371 Rekow patent, but without the need for physical stops. A potential problem presented by the microlens arrays, however, is that light can be scattered at boundaries or edges of the microlenses. This can provide light loss in the projected line comparable with losses due to stops in the '371 Rekow patent. It has also been found difficult, in manufacturing, to reproducibly align the microlens arrays with diode-laser arrays having a different spacing.